BCA Class and Object (3).pptx

Class
• Class is a logical entity because when we are creating a class then
memory will not be created for a class.
• Class is a collection of data members, constructors, methods and
blocks.

Cont…
• Class is a collection of objects of same type.

Objects
• Object is a physical entity because when we are creating object then
memory will be allocated for object.
• Object is created with the help of new keyword.

Cont…
• Object is the instance of a class.

Constructors in Java
• In Java, a constructor is a block of codes similar to the method.
• It is called when object is created.
• At the time of calling constructor, memory for the object is allocated
in the memory.

Cont…
• It will be used to initialize the object.

Rules for creating Java constructor
• Constructor name must be the same as its class name.
• A Java constructor cannot be abstract, static, final, and synchronized

Types of Java Constructors
• There are 3 Types of constructors.
• Default Constructor.
• No-argument Constructor.
• Parameterized Constructor.

1. Default Constructor.
• If there is no constructor in a class, compiler automatically creates a
default constructor.

Example
class Student
{
int id;
public static void main(String a[])
{
Student s1=new Student();
System.out.println(s1.id);
}
}
Output: 0

2. No-argument Constructor.
• A constructor that has no parameter is known as no-argument
constructor.

Example
class Bike
{
Bike()
{
System.out.println("Bike is created");
}
{
Bike b=new Bike();
}
}
Output: Bike is created

3. Parameterized Constructor
• A constructor which has a specific number of parameters is called a
parameterized constructor.

Example
class Student
{
int id;
String name;
Student(int i, String n)
{
id = i;
name = n;
}
void display()
{
System.out.println(id);
System.out.println(name);
}
{
Student s1 = new Student(111,"Karan");
s1.display();
}
}

Difference between constructor and method
in Java
Constructor Method
A Constructor is called implicitly by the
system.
A Method is called by the programmer.
A Constructor is called when a object is
created using the keyword new.
A Method is called through method calls.
A Constructor’s name must be same as
the name of the class.
A Method’s name can be anything.
A Constructor doesn’t have a return type. A Method must have a return type.

Ways to initialize object:
There are 3 ways to initialize object in Java.
• By reference variable
• By method
• By constructor

1) Initializing an Object by reference variable
• Initializing an object means storing data into the object.

Example
class Student
{
int id;
{
Student s1= new Student();
s1.id=101;
System.out.println(s1.id);
}
}
Output
101

2) Initializing an Object by method

Example
class Student
{
int id;
void display(int n)
{
id = n;
}
{
Student s1=new Student();
s1.display(101);
}
}

3) Initializing an Object by a constructor

Example
class Student
{
int id;
Student(int i)
{
id = i;
}
void display()
{
}
{
Student s1 = new Student(101);
s1.display();
}
}

Java static keyword
• The static keyword in Java is used for memory management mainly.
• We can apply static keyword with variables, methods and blocks etc.

Cont…
• The static can be:
• Variable (also known as a class variable)
• Method (also known as a class method)
• Block

1) Java static variable
• If you declare any variable as static, it is known as a static variable.
• The static variable can be used to refer to the common property of all
objects (which is not unique for each object).
• for example, college name of students, etc.

Cont…
• The static variable gets memory only once in the class area at the time
of class loading.

Advantage of static variable
• It makes your program memory efficient (i.e., it saves memory).

Example: Understanding the problem without
static variable
class Student
{
String collegename;
Student(String c)
{
collegename = c;
}
void display ()
{
System.out.println(collegename);
}
{
Student s1 = new Student("GLAU");
Student s2 = new Student("GLAU");
s1.display();
s2.display();
}
}

Example of static variable
class Student
{
static String collegename =“GLAU";//static variable
void display ()
{
System.out.println(collegename);
}
{
Student s1 = new Student();
Student s2 = new Student();
s1.display();
s2.display();
}
}

2) Java static method
• If you apply static keyword with any method, it is known as static
method.
• A static method can be called without the need for creating an instance
(object) of a class.

Restrictions for the static method
• There are two main restrictions for the static method. They are:
• The static method can not use non static data member or call non-static method
directly.
• this and super cannot be used in static context.

Example of static method
class A
{
static void display()
{
System.out.println("Static Method");
}
}
class B
{
{
A.display();
}
}

3) Java static block
• It is used to initialize the static data member.
• It is executed before the main method at the time of class loading.

Example of static block
class A
{
static
{
System.out.println("static block");
}
{
System.out.println("Hello main");
}
}

Cont…
Output:
static block
Hello main

this keyword in java
• this is a reference variable that refers to the current class
object.

Usage of java this keyword
• There are many usage of java this keyword.
• this can be used to refer current class instance variable.
• this() can be used to call current class constructor.

1) this: to refer current class instance variable
• The this keyword can be used to refer current class instance
variable.
• If there is ambiguity between the instance variables and
parameters (local variable), this keyword resolves the
problem of ambiguity.

Example: Problem without this keyword
class Student
{
int x;
int y;
Student(int x, int y)
{
x = x;
y = y;
}
void display()
{
System.out.println(x);
System.out.println(y);
}
{
Student s1=new Student(10,20);
s1.display();
}
}

Ex: Solution of Previous Problem with this keyword
class Student
{
int x;
int y;
Student(int x, int y)
{
this.x = x;
this.y = y;
}
void display()
{
System.out.println(y);
}
{
Student s1=new Student(10,20);
s1.display();
}
}

2) this() : to call current class constructor
• The this() can be used to call the current class constructor.
• Call to this() must be the first statement in constructor.

Example 1
class A
{
A()
{
System.out.println("hello a");
}
A(int x)
{
this();
}
public static void main(String a1[])
{
A a=new A(10);
}
}

BCA Class and Object (3).pptx

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